1. The field of art to which the invention pertains includes the art of valves and particularly to diaphragm-type valves for high pressure, high temperature line service.
2. The need for reliability against stem or bonnet leakage in an in-line valve is known to increase in correlation with the pressure and temperature service for which the valve is to be used. When operating under severe service conditions on the order of 2000 p.s.i. and 650.degree. F., it is common to use either a bellows-seal construction or a multi-layer diaphragm construction in combination with stem packing in order to prevent line leakage past the stem cavity. Of the two, the bellows seal construction is usually the more costly by a significant margin. Contributing to lower cost of the diaphragm valve is the usual employment of a relatively uncostly globe valve body arrangement able to readily accommodate lift limitations associated with the diaphragm and constructed preferably in a Y-pattern to enhance flow characteristics. Conventional globe type Y-pattern valves of this type are available from a variety of commercial sources. A disclosure of the latter construction is contained in U.S. Pat. No. 3,874,636.
While the diaphragm valve has enjoyed a high order of commercial success, one characteristic deficiency has been the relative short life expectancy under severe operating service of the diaphragm per se to on the order of 500-3000 open and closing cycles. This is generally attributed to the variety of stresses imposed under pressure against the diaphragm in being flexed between open and closed positions of the valve. With each complete open and closing cycle, at least the central portion of the diaphragm incurs a reverse shaping distortion between convex to concave as the valve disc is moved toward and away from seating relation tending to cause such wear phenomena as buckling, creasing, fatigue, etc. Should diaphragm failure be detected, it is customary to shut down operation in order to effect a maintenance replacement therefor, but if undetected only the stem packing remains as a deterrent to leakage. With the advent of nuclear service to replace fossil fuel for power generation, the increased operational severity and maintenance difficulties has rendered it increasingly imperative to hold operational shutdown to a minimum. As a consequence, valve standards for some nuclear services now require that the diaphragm have a minimum life expectancy of at least about 12,000 cycles. This standard, of course, far exceeds prior art capability and despite recognition of the problem a suitable construction able to conform with the requirement has not heretofore been known.
Provided as a backup to prevent leakage in the event of diaphragm failure in these valves is the packing compressed in the bonnet base about the valve stem. In the usual construction to effect adequate pack there is included a packing gland, gland flange, gland bolts, nut, etc. employed in combination with a forged or cast bonnet yoke. While the high cost attributed to this assembly combination has long been known, it has heretofore been unknown how to significantly reduce costs without sacrificing the quality and reliability that this construction has afforded.
At the same time, diaphragm valves of the type contemplated herein, particularly in sizes 2 inches and greater typically imposed highly complicated, confined and multi-directional flow patterns past the valve disc. Because lift dictated by use of a diaphragm is relatively small, a larger than normal seat diameter is required to obtain the necessary flow area. This in turn causes the flow path to incur a sharper than normal bend in and about the seat contributing to a high pressure loss and consequently a lower than desirable coefficient of flow (Cv). A 2-inch full ported valve of this type usually has a Cv in the range of about 40-60. Despite various approaches to the problem these valves have been generally unable to further increase the capacity rating beyond an upper limit of about 60.